Wave energy has been identified as one possible source of renewable energy. Various wave energy conversion devices have been proposed that aim to extract useful energy from wave motion in a body of liquid, such as the sea. Waves are created through the transfer of wind energy to the surface of bodies of water. Wave energy is propagated for long distances in deep water with minimal attenuation by interactive velocity and pressure fluctuations within the body of water. Fifty percent of the energy is propagated by pressure fluctuations and fifty percent by velocity fluctuations. A successful wave energy conversion device must maximise interaction with these energy vectors, which are continually changing over time intervals spanning from seconds to years. Known wave energy conversion devices generally fall into two categories, namely wavelength devices and point absorbers.
Wavelength devices physically span the waves from crest to crest to provide a floating reference for the wave displacement forces, and are therefore very large structures of at least 100 to 200 meters in length. It is difficult to design a cost effective device of this size which can respond to short term changes in wave characteristics, and so large devices tend to have low overall conversion efficiency. Point absorbers are smaller and potentially more adaptable to changing wave conditions and so can have higher practical efficiencies. They use some other form of reference, for example the ocean floor or the shoreline, or through sufficient inertia in one component of the system. There are many different types of point absorber, including oscillating water columns and heaving buoys.
Oscillating water column based devices typically utilise a vertical tube or chamber in which waves surge back and forth to drive air through a power conversion device such as an air-turbine.
Heaving buoy point absorbers usually operate in a vertical mode, often referred to as “heave”, and typically utilise a surface float that rises and falls with passing waves and reacts against the seabed or the inertia of a connected component to extract useful energy. A common type of point absorber is known as a self-reacting heaving buoy system that utilises a heaving buoy which reacts against a drag plate suspended and submerged below the active wave energy region, or an internal inertial mass which is designed to resonate at the wave period. There are essentially the following three basic components to a self-reacting heaving buoy system: a heaving buoy floating on the surface of the sea, some form of reaction device suspended and submerged below the surface, and a load resistance or power take-off placed between them. Some power take-off techniques limit adaptability to wave conditions and defeat the inherent responsive benefits of point absorbers. Direct drive point absorbers minimise these limitations as the response characteristics of the device can be largely controlled by the power take-off loading applied to the device.
Variations of the basic self-reacting heaving buoy systems described above have also been proposed. For example, U.S. Pat. No. 6,857,266 describes a wave energy converter that extracts useful energy based on the relative movement between two devices, each device comprising a surface float and a submerged body. The relative movement of the two devices in response to a passing wave may be used to effect an energy transfer as the devices are connected by linkages that are typically arranged to operate as power take-offs, for example they may be hydraulic devices that are arranged to drive an electrical generator.
Another type of point absorber based design, known as a “heave and pitch buoy”, is described in Japanese patent 3218462. The heave and pitch buoy is arranged to extract useful energy from heave motion and also from rotational horizontal motion, often referred to as “pitch”, caused by the exponential reduction in energy with depth of waves. In particular, the passing waves excite the heave and pitch buoy to oscillate in two directions, namely vertically for heave motion and horizontally for pitch motion. More specifically, heave motion is created in the vertical direction by potential wave energy and pitch motion is created in the horizontal direction by kinetic wave energy. The heave and pitch buoy has two floats, a main float and a sub-float. Plates join the two floats together and a link arm connects them to a reference tower which is fixed to the seabed. The link arm is pivotally connected to the tower at one end and pivotally connected to the main float at the other end. The main float is arranged to extract wave energy via its heave and pitch motions. In particular, the main float is designed to resonate with the waves in the heave and the heave of the sub-float also causes the main float to pitch. The heave and pitch motions are detected by the relative rotary displacement at the two pivotal connections which are in turn coupled to two rotary vane pumps so that the motion can converted into oil-hydraulic power for an oil-motor to drive an electrical generator.
Another type of point absorber based design is proposed in U.S. Pat. No. 3,631,670. The device proposed comprises two floating bodies that are linked by a rocking rod. In operation, the two floats bob up and down in the water at different frequencies and this imparts a rocking motion on the rocking rod. The device also comprises a mechanism for converting the rocking motion of the rocking rod into rotary motion that can be used to drive an electric power generator.
In this specification where reference has been made to patent specifications, other external documents, or other sources of information, this is generally for the purpose of providing a context for discussing the features of the invention. Unless specifically stated otherwise, reference to such external documents is not to be construed as an admission that such documents, or such sources of information, in any jurisdiction, are prior art, or form part of the common general knowledge in the art.
It is an object of the present invention to provide an improved wave energy converter for extracting useful energy from wave motion in a body of liquid, or to at least provide the public with a useful choice.